Retroviral vectors have been used to stably express high levels of COX-1 or -2 in both the C3H10T1/2 and AS52 cell lines. In a series of substrate metabolism studies, COX-1 in these cells was better able to metabolise exogenous 10fM arachidonic acid (AA), while COX-2 was better able to use the endogenous AA released by phorbol ester (TPA). The two isoforms were about equal in their ability to use arachidonic acid released by calcium ionophore. In a further series of experiments, it was found that NSAIDs differentially inhibit the cyclooxygenases depending upon the whether source of AA is endogenous or exogenous. The rank order of the IC50 ratios (the potency of inhibition by an NSAID for COX-2 relative to COX-1) with endogenously released AA were more closely correlated to the relative therapeutic/toxicologic effects of NSAIDs in vivo than were the ratios derived with exogenous AA as substrate. These cells have been used to demonstrate the metabolic activation of 1,1- dimethylhydrazine, aflatoxin B1, and N-acetylbenzidine by cyclooxygenase. Recent data from collaborative studies indicate a cytotoxic effect by phenolphthalein in COX-2 expressing cells, but not in those expressing COX-1. The studies demonstrate the need for conducting metabolism/inhibition experiments in whole cells of similar origin that are capable of sustained high levels of cyclooxygenase expression.